I agree with other posters who said that there's too little information to go on.

I'm a little gun-shy about mechanics looking for alignment problems in frames. Most frames these days come from the factory accurately aligned, and accidents that are severe enough to bend the frame but not severe enough for the bending to be obvious without measurement are somewhat rare. The average mechanic doesn't see many such frames and thus might be prone to performing or interpreting the measurements incorrectly.

In short, I'd be happy to trust a framebuilder's judgment about a possibly bent frame; I'd be less trusting of the average mechanic. (I've had mechanics working in shops that I ran who learned the rudiments of frame alignment measurement techniques and then started claiming that every other frame in the shop was misaligned by their measurement. They weren't.)

Was the shifting good earlier in your ownership of the bike? If so, have you recently had an accident with the bike that was extreme enough to bend the frame? If not, I'd be reluctant to use that particular bigger-hammer route before the list of simpler fixes has been exhausted.

Did the mechanic check the dropout alignment to confirm whether or not the derailleur mounting tab is square with the rest of the frame? If not, get it checked.

Also, if your mechanic didn't try squirting WD-40 into the shifters to free up the gunk that develops in the inner mechanism, I'd try that. It's a quick fix if it helps, and it can't hurt.